[camlorn]

I think that it wouldn't work for players actions happening "now", because said actions happen at the instant of the enter key. Using said makes it seem like the events have already happened, which presumes that the ending is already known. To me, this would be jarring, and I don't think I'd gain much out of it; that said, I'm not into RP. The specific problem here is that past tense implies that I know the story already or that a narrator is telling it to me, not that I am actively participating or that my actions are meaningful. It would cause a general disconnect, not a specific, focused one, and I don't think that's a good thing. I think it might have a place in IF, of some sort, perhaps for dream sequences or places where the player never really had a choice. Past tense is the wheel of fate, grinding forever forward, grinding us to dust beneath it's inexorable force, and we have no choice.
Using this as flashbacks, or to record RP events, though. That's actually a cool idea. I imagine that you could write present and past tense versions of all the socials, that says can be converted to said and the like, that generally converting any predefined block of text into the past tense shouldn't be a problem. Given a dictionary library and a bit of clever coding, converting even freeform emotes should work out, and the actual content of says doesn't need to be converted. I think that, if one is looking to use past tense, this is the place.
Perhaps I'm completely wrong, though.